There is a strong interest in organic light emitting displays (OLED) because of the properties of organic light emitting devices. Generally, these devices are of very low current, low power, and high emission characteristics. Further, organic light emitting devices can be produced to emit virtually any color so that color displays are possible. As is understood by those skilled in the art, a color display requires arrays of full-color pixels each of which includes red, green, and blue sub-pixels. However, it is very difficult to fabricate organic light emitting devices in arrays of color pixels. At present the only practical method is to deposit the various layers of color material required by using a process known as ‘fine shadow masking’ or the use of a shadow mask to deposit the patterned color emitter layers. The major problem is that this shadow mask is very difficult to make and expensive. Secondly, this shadow mask can only be used for certain deposition cycles due to dimension deformation. Moreover, the shadow mask process has upper size limits that restrict the process to relatively small displays. On the other hand, displays for 3G or 4G multi-medium applications require sufficient pixel counts for each display, the mask alignment accuracy and the corresponding emitting color crosstalk set a limit to the size of red, green, and blue sub-pixels.
High information content color arrays use an active matrix type of pixel control and address system. Generally, because the controlling transistors are built into the array, thin film transistors (TFT) are used. In the prior art, poly-silicon is used for the switching and control transistors in active matrix OLED displays (AMOLED). However, poly-silicon requires relatively high temperatures to process, and, therefore, adjoining circuitry and substrates are severely limited. Also, the characteristics of transistors formed in poly-silicon can vary, even between adjacent devices in an array, because of the variation in crystal size and position. To better understand this problem, in a conduction area under a gate of a few microns length each different transistor can include from one or two poly-silicon crystalline grains to several crystalline grains and the different number of crystals in the conduction area will produce different characteristics. The dimensions and their physical characteristics among different grains are also different. In addition, poly-silicon is light sensitive, i.e. its I-V characteristic is changed by exposure to visible light. Amorphous silicon is also light sensitive so that devices fabricated from either of these materials require a light shield or light shielding, which further complicates the manufacturing process and reduces the aperture ratio (the emitting area over the pitch area). Small aperture ratio, in turn, requires the OLED be driven harder for a targeting display brightness and, thus, sets higher demand to OLED operation lifetime.
Fundamentally, a pixel driver for an active matrix organic light emitting display includes two transistors and a storage capacitor. One transistor serves as a switch and the other transistor serves as a current regulator for the OLED. A storage capacitor is connected between the gate and the drain (or the source) of the current regulator transistor to memorize the voltage on the data line after the switching transistor is turned off. Also, the pixel driver is connected to three bus lines, a scan or select line, a data line, and a power line, which are coupled to peripheral control circuitry. However, in the prior art, or the present state of active matrix organic light emitting displays, the pixel driver described cannot effectively be achieved with sufficient performance and/or at low cost.
Low temperature poly-silicon (LTPS) and amorphous silicon (a-Si) have been used to construct pixel driver circuits for active matrix organic light emitting display backpanels. In this context, the term “backpanel” refers to any array of switching circuits, generally arranged in column and row form, and each pixel or pixel element having a pixel electrode (either transparent or reflective to the emitted light) connected to an organic light emitting diode. At the present time all of the active matrix organic light emitting displays in the commercial market are fabricated with LTPS backpanels. Although LTPS provides sufficient operating lifetime needed for driving OLEDs, the “mura” defect caused by TFT performance inhomogeneity is much more serious for LTPS TFTs being used for driving OLEDs than for driving liquid crystal displays (LCDs). As a result, more than 2 transistors are often used in pixel drivers to compensate for the mura inhomogeneity. Also, in some applications more than three bus lines (data, selection, and power) are included for compensation circuitry. Further, LTPS backpanels require larger storage capacitors due to relatively higher “OFF” current in the switching transistors. Although higher mobility in LTPS backpanels allows transistors with shorter width/length (W/L) ratio, the higher OFF-current in the switching transistors requires multiple gate design (e.g. a TFT with 2 or 3 gate electrode in comb pattern between source and drain electrodes) and thus larger space between source and drain electrodes. Thus, the effective area needed for each pixel driver is substantial compared to the total pitch area. Thus, the OLED emitter has to be arranged or stacked with the pixel driver for light emission from the top. The small energy gap of LTPS also requires that LTPS TFTs are shielded from the light being emitted as well as from ambient light.
There has been significant effort to fabricate active matrix organic light emitting display backpanels using a-Si TFTs. However, the I-V (current-voltage) performance in a-Si TFTs is not stable under DC operation (Vth shift and mobility decrease due to defect density increase) so that it is hard to use a-Si TFTs for the driver or current regulator transistor in the backpanel. Pixel control circuits with more transistors, capacitors, and buslines have been proposed to stabilize the transistor performance but none of them have demonstrated the stability needed for commercial applications. The low carrier mobility (˜0.1 to 0.7 cm2/Vsec) also requires larger W/L ratio (and thus larger TFT size) for the driver or current regulator transistor. As a result, there is not sufficient room for an OLED emitted pad for a bottom emission so that a top emission configuration has to be used.
In the top emission active matrix organic light emitting display design, a planarization layer is needed to separate the TFT from the bottom electrode of the OLED emitter to eliminate optical and electrical crosstalk between the two portions. Two to three photo processes are needed to make vias through the planarization layer and to pattern the bottom electrode for the OLED. There is often another 1 to 2 photo process steps to construct a bank structure for full-color OLED processing (such as a well used to confine organic emitter ink when inkjet printing is used to pattern full-color emitter layers). Since the bulk conductivity of transparent top electrode (typically made of indium-tin-oxide or aluminum-zinc-oxide) is not sufficient for the common electrode to pass current from pixels to peripheral driver chips, another via is often needed and another metal bus line is thus needed on the backpanel. This design severely limits the aperture ratio of the top emission active matrix organic light emitting display products to ˜50% range. Here “aperture ratio” means a ratio of emission zone over the sub-pixel pitch size. Moreover, depositing transparent metal oxide onto OLED layers is typically done by a sputtering process, retaining OLED performance (both power efficiency and operation lifetime) in top electrode process is one of the remaining challenges.
There is, thus, a strong interest in a bottom emission active matrix organic light emitting display architecture with sufficient aperture ratio for the OLED pad, and with low cost processes at least competitive to other display technologies, such as active matrix liquid crystal displays.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved full-color, active matrix organic light emitting display.
It is another object of the present invention to provide a new and improved full-color, active matrix organic light emitting display which is relatively simple and inexpensive to manufacture and which results in higher fabrication yields.
It is another object of the present invention to provide a new and improved full-color, active matrix organic light emitting display utilizing pixel control circuits with relatively uniform characteristics.
It is another object of the present invention to provide a new and improved full-color, active matrix organic light emitting display that can be constructed with relatively large areas.
It is another object of the present invention to provide a new and improved full-color, active matrix organic light emitting display that can be constructed with high pixel density.
It is another object of the present invention to provide a new and improved full-color, active matrix organic light emitting display with bottom emission and relatively high aperture ratio.
It is another object of the present invention to provide a new and improved full-color, active matrix organic light emitting display requiring low cost processes at least competitive to other display technologies.
It is another object of the present invention to provide a new and improved full-color, active matrix organic light emitting display with higher switch rate and higher frame rate beyond 60 Hz (higher carrier mobility in MO-TFT enables the frame rate to 120 Hz or beyond).
It is another object of the present invention to design a full-color, AMOLED with optimized energy efficiency, color gamut, operation lifetime in addition to the lowest manufacturing cost and highest manufacturing yield, i.e. a full-color AMOLED with the best performance/cost ratio.